1. Field of the Invention
The present invention relates to a paper folding device for tri-folding sheets of paper.
2. Description of the Related Art
Conventionally, various techniques have been disclosed in relation to folding devices making two folding lines to provide tri-fold paper.
By way of example, Reference 1 (Japanese Laid-Open Patent Publication No. 05-319685) discloses a paper folding device executing two-fold mode and three-fold mode using first to third rollers. In the two-fold mode, a sheet of paper is fed by first and second rollers with feeding speed made different between the rollers, so that the sheet is curved. Then, the sheet is further fed with the curved portion carried between the first and third rollers. In the three-fold mode, after the curved sheet is carried between the first and third rollers, the feeding speed is made different between the first and third rollers so that the sheet is again curved, and the sheet is further fed with the curved portion carried between the first and second rollers.
Reference 2 (Japanese Laid-Open Patent Publication No. 2003-146534) discloses a paper post-processing device capable of executing saddle stitch processing, saddle fold processing and three-fold processing modes. In the paper post-processing device, based on the combination of process mode and paper size, stop position of position regulating means, to which the sheet of paper abuts and is positioned thereby, is determined in accordance with the combination of process mode and the size of paper loaded on a paper tray.
Reference 3 (Japanese Laid-Open Patent Publication No. 2002-220154) discloses a technique of folding a bundle of paper in three, using a folding knife, in a book binding machine. The process of folding a bundle of paper to three includes two steps of folding. Let us call the first step as the first folding step and the second step as the second folding step. In the book binding machine, the folding knife is used in the first folding step, and only the rollers are used in the second folding step. Positions of the rollers used in the second folding step are changed in accordance with the thickness of the bundle of paper.
Reference 4 (Japanese Laid-Open Patent Publication No. 2002-060127) discloses a technique that allows selective use of three-folding and two-folding functions in a post-processing device. In the three-folding function, a folding knife is used in the first folding step, and only the rollers are used in the second folding step. Reference 4 further discloses a technique of forming an image appropriate for the folding process on a sheet of paper, in an image forming apparatus combined with the post-processing device.
Reference 5 (Japanese Laid-Open Patent Publication No. 2001-341937) discloses a post-processing device in which folding knives are used in the first and second folding steps.
In the paper folding device, if the number of sheets as the object of one folding operation increases, problems such as crease generated during paper feeding or loose folding can better be avoided by using the folding knife, rather than using rollers only in the folding step. Therefore, it is preferred that the folding knife is used not only in the first folding step but also in the second folding step.
In Reference 5 in which the folding knives are used in the first and second folding steps, the timing of feeding the bundle of paper or the timing of operation of the folding knives are not mentioned.
For instance, the second folding step should be started after the end of the first folding step, when it is detected that the paper has reached an appropriate feeding position. In this regard, it is possible to determine the start timing of the second folding step by providing a sensor appropriately in the device. Provision of a sensor, however, leads to increased size and more complicated structure of the device, as well as higher manufacturing cost.